casting technology'.ppt
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Casting TechnologyTRANSCRIPT
م�ن عق�د�ة� لل� و�اح�ان�ي {27}ل�س�
ق�و�ل�ي �ف�ق�هوا {28}ي
ح�من� �الر الله� � م �س� ب
�� ح�يم � الر
ا العظيم صدق
Casting Technology
The purpose of the casting procedure is to provide exact metallic duplicate of missing tooth (or teeth) structure with
maximum possible accuracy in all dimensions, For →
InlaysOnlays
Crowns & Bridges
Direct restoration
Indirect restoration
InlayOnlay
Gold bridge
Reduced tooth
Impression
Cast / Die
Gold Crown
Steps of casting procedures
Wax pattern constructionSpruingInvestingWax eliminationCastingFinishingPolishing
I- Wax pattern construction
Direct technique
Indirect technique
Indirect direct technique
II- Spruing
Create a channel to allow the molten wax to escape from the mold.
Enable the molten alloy to flow into the mold which was previously occupied by the wax pattern.
Role of a sprue:
The wax pattern is attached to a sprue
for proper handling without distortion
Sprue materials
Waxes
melt at the same rate as the wax pattern thus allowing easy escape of the molten wax
Plastics
MetalsHollow
Or
solid
Attachment and Direction: Sprue should be attached to the
thickest part of the wax pattern
with a 45º C angle to the proximal wall
This orientation avoids:
* Turbulent flow of the molten metal
→ air bubbles leading to
# porosity
Diameter (Thickness):
Equal or slightly Thicker than the thickest part of the wax pattern →
* improves flow of alloy into the mold • provides reservoir during solidification.
Reservoir: piece of wax attached 1 mm. Away from the wax pattern it is the last part to solidify,allows continuous feeding of molten alloy to compensate solidification shrinkage and to avoid shrinkage porosity
Number of spruesmay be single, double or multipleThere may be a main sprue with accessory sprues.
Single Double Multiple
Length of sprueShould keep the wax pattern
about 6-8 mm from the end of the casting ring
Short sprue → prevent escape of air leading to
back pressure porosity.Long sprue → fracture of the investment, as mold will not withstand the impact force of the entering molten alloy.
Preparation of the casting ring
Crucible former: usually made of rubber, which constitutes the base of the casting ring during investing.
Casting ring and liner: Casting ring may be metallic, or rubber ring.
Selection depends on: choice of casting alloy and investment material.
Casting ring lined using asbestos,
then wet the liner.
Functions of liner:
1) Creates space to allow for investment expansion.
2) Allow setting of investment material under water to give hygroscopic expansion.
3) Facilitate removal of investment from the casting ring after casting.
Types of ring
Metallic Rubber Split
III- Investing
Surfactant
Investing the wax pattern:Wax pattern must be invested immediately to avoid distortion of wax pattern,
which may be due to release of internal stresses
Wax is a hydrophobic material → Improper wetting of wax pat. →
irregular surface of the metallic cast. Minimized by applying
surfactants (surface acting agents)
on the wax surface → * increases surface energy of the wax * decrease contact angle between and
investment material.
Methods of investing the wax pattern: Hand investing procedure and vacuum ***Vacuum mixing of investment
is highly recommended to obtain casting with minimal surface defect.
Investing wax pattern may be
done by regular mix investment technique OR by double mix investment technique → * improve adaptability of the investment material to the surface of wax pattern * avoid distortion of wax
# After the ring is filled to the rim, # allow the investment to set for about one hour # remove the crucible former and the metallic sprue
Investing technique
Hand mixing Vaccum
IV- Wax elimination
Wax elimination: Heating the investment must be done
to achieve the following:1) All traces of the wax are vaporized: bring
the furnace to 200º C and hold this temperature
for 30 minutes . Most of the wax will be eliminated by this
time.2 )Increase the temperature gradually to the
final burnout temperature (about 650º C), this is accompanied by thermal expansion of the investment.
The mold temperature is now ready for casting.
V- CastingA) Melting the alloy
Using flame Using electric furnace
B) Casting:A casting machine requires * a heat source to melt the alloy. * casting force, so the casting will be completed.
Methods of melting alloys:* flame: Melting gold alloys is done by a blow pipe flame while melting base
metals is done by oxyacetylene flame.
* electric furnace.Methods of Casting: (driving the alloy into the mold)* Air pressure casting machine Compressed air* Centrifugal casting machine.* Both centrifugal and gas pressure
Casting machines
Air pressure casting machine Centrifugal casting machine
Solidification of the alloy is done under pressure, then
Allow for cooling and
when the color of the molten alloy has changed
from red to black color,
the ring should be quenched in water.
Devesting , Finishing and polishing of the casting are subsequently performed.
VI- Divesting and finishing
VI- Polishing
Casting Defects
I) Distorted casting: Due to distortion of the wax pattern.
** Improper handling of the wax pattern and/or its
** Delayed investing.
II) Dimensional inaccurate casting: Small or large casting may be due to :
a. Improper type of investment material.
b. Improper temperature.
III) Surface roughness of the casting: due to:
*** Air bubbles on the surface of wax pattern during the investment procedure
* Improper use of wetting agent
* Improper vacuum investing
VI) Porosity:
1) Shrinkage spot porosity:
at the point of attachment to the sprue, it is due to using a
thin sprue,
which leads to incomplete feeding of the molten alloy during solidification.
2) Back pressure porosity: It is due to using
***short sprue. The distance between the wax pattern and the end of the ring is more than 6 mm;
***Non porous investmentair will not escape from the mold and creates back
pressure during flow of molten alloy into the mold.
V) Incomplete casting and rounded margin:
Due to resistance of the flow of molten alloy into the mold:
*Using short sprue.
*Insufficient venting of the Investment.
*Insufficient casting pressure.
*Under heating of the alloy causing premature solidification.
*Incomplete wax elimination.